The present invention generally relates to sensors and their fabrication. More particularly, this invention relates to a sensor construction and fabrication technique capable of yielding an oxazine-based sensor suitable for detecting contaminants in stagnant and dynamic fluid media.
Ammonia at elevated concentrations is poisonous to various organisms. As examples, ammonia at a level of about 22.8 ppm can be lethal to water organisms, and exposure to ammonia at levels as low as 35 ppm for more than fifteen minutes can be dangerous to human beings. Therefore, ammonia monitoring systems capable of continuous online operation are highly desired. A particular but nonlimiting example is the detection of ammonia in dynamic aqueous systems, including municipal water systems.
While various technologies have been studied for the detection of ammonia, sensors such as electro-chemical gas sensors, catalytic sensors, chemi-resistor sensors, CHEMFET (chemical field effect transistor) sensors, and optical fiber sensors have attracted substantial attention for their ability to rapidly and continuously detect levels of gaseous phase ammonia. Among these sensors, optical fiber chemical sensors are of particular interest because they tend to be compact, light-weight, relatively inexpensive, easily multiplexed and immune to electromagnetic interference (EMI), and do not require electric power at the sensing point.
The operation of optical fiber chemical sensors is based on the ability of micro or nano-structure materials to alter their optical responses (such as reflection or absorption) in the presence of a “recognition element.” Experimentation has been conducted with optical fibers and especially glass optical fiber (GOF) or optical waveguides as sensing elements. The majority of optical fiber sensor research for the detection of ammonia is believed to have been focused on the employment of GOF, and primarily focused on the detection of gaseous phase ammonia (for example, in air), and to a lesser extent the detection of liquid phase ammonia (for example, in aqueous media). GOF sensors have been developed that employ various different sensing materials, for example, polyaniline/poly(methylmethacrylate), bromocresol purple (5′,5″-dibromo-o-cresolsulfophthalein)-based silica-solgel, polyaniline-based silica-core, silica solgel-based oxazine 170 perchlorate/cellulose acetate, and bromothymol blue (BTB)/TiO2 (tin oxide) films for the detection of ammonia in air. Though polymer optical fibers (POF) offer certain advantageous characteristics, for example, greater flexibility and mechanical strength as compared to GOF, it is believed that much less research has been conducted toward their use in the detection of ammonia.
Regardless of whether the intended media is gaseous (for example, air) or liquid (for example, water), a cost-effective sensor must be capable of exhibiting a high level of reversibility, which as used herein refers to the ability of a sensor to return to its initial condition with little or no hysteresis upon the removal of the analyte. GOF sensors that employ the aforementioned sensing materials have been often observed to exhibit sufficiently high hysteresis to be deemed to have poor reversibility. In contrast, optical sensors based on glass capillary tubes coated with oxazine 170 perchlorate (C21H22ClN3O5; CAS No. 62669-60-7) have been reported to exhibit good reversibility. A notable property of oxazine 170 perchlorate is its optical responsiveness to bandwidths of 455 to 595 nm, which allows for multiplexing. However, while various coating methods have been developed for silica-based GOF sensors, for example, dip coating, spin coating, and spray coating, available literature does not appear to indicate the successful deposition of oxazine 170 perchlorate on other optical fiber materials, for example, POF materials.